You only need enough tension on the main line  just to hold the wraps in place while you wind the knot up ,  otherwise the knot gets a bit lumpy , doesnt seem to affect the strength ( never had one come undone )   just dont look so good , have a butchers at a finished knot  with a magnifying glass.......the whole plot becomes a bit clearer when you see how  the finished knot wraps the main line around the leader

 I found that the position of the cannon  should be  pretty much adjacent to the spool , to far up the rod  dont work .

The other thing I learnt was that  the amount of oomph  one puts into the cast  verses the distance the rig flies is one of them diminishing return things , and by putting in half the effort the rig will go 3/4 the same distance without degloving your finger and anyhow  most of the fish are not that far off shore.

 Im fairly new to surfcasting   but I have 50lb braid and 2 rod lengths of  30lb fluro , and I did find the cannon needs a bit of practice and I aint sure (yet )  if Im going to keep using it . If you really need the distance its probly worth persevering  

Check out some vids of  long distance casting competitions ..... these guys make it look easy .